1. Field of the Invention
This invention relates to a process of fabricating a semiconductor device, and more particularly to a method of processing a dielectric film using photolithography.
2. Description of the Prior Art
As well-known, photolithography involves selectively patterning a photoresist film coated over a wafer on the same principle as photography, and using the patterned photoresist film as a mask for etching, and/or introducing impurities through to an exposed wafer surface. For realization of scaling down of semiconductor devices which is one of trends these days, one of the most important keys is, as well known, to make the photoresist pattern more precise without losing the masking ability.
The photolithography process in the prior art will be described by way of an example of processing a cover film of the semiconductor memory device.
A silicon substrate on which circuit elements such as transistors are built is covered with a dielectric layer as of silicon oxide film, and then through-holes and others structures are formed. On the dielectric layer an Al-Si alloy film is deposited to a 1 .mu.m thickness, followed by patterning to form a number of word lines, 1 .mu.m wide and about 1 mm long in parallel arrangement with a 1.2 .mu.m spacing. A 1 .mu.m-thick silicon oxynitride film is deposited over the resulting structure by plasma CVD technique to form a passivation film. The passivation film thus formed has a poor step coverage, which allows easy formation of voids between the word lines, such as cave-like voids having an opening at each end of the word line group. There is a tendency toward formation of, so to speak, roof windows at the right-angle bends of word lines, and hence in the case where any word line ends there, a larger opening is produced. A photoresist film is deposited on the wafer by a spin-on technique. Then the photoresist is forced to flow into the voids, especially, cave-like voids, which as a result become bubbles. After exposure and development, the passivation film is etched, accompanied by a thinning of the photoresist film, which forces the voids to break. Near these, unwanted etching and the consequent damage of the passivation film result.
As described above, with scaling down of the semiconductor device, the topology of the photolithography-processed substrate surface becomes complicated. In this circumstances, deposition of the dielectric film by CVD technique may be accompanied by formation of voids. This leads to poor masking ability of photoresist film for processing insulating films, inevitably only damaged dielectric layers and cover films being formed. There is danger associated with such a dielectric layer to bring about insulation failure. The use of such a cover film may be the cause of lowering the reliability under severe conditions of high temperature and humidity even if operation of the semiconductor device itself is unaffected in an ordinary environment.